Abstract

Regeneration of portions of the crinoid endoskeleton following loss to predation attempts or autotomy is a well-known phenomenon. To date, however, most effort has focused on patterns, frequencies, and evidence of regeneration of arms and portions of the calyx, with few reports of unusual or significant regeneration-related features preserved in isolated ossicles. Here we describe brachial spines belonging to undetermined pirasocrinid (cladid) crinoids from the Upper Pennsylvanian Ames Member of the Glenshaw Formation of east-central Ohio that display evidence for multiple episodes of breakage and regeneration. As evidenced by major size discontinuities along the length of single spines, some specimens regenerated at least two times during the lifespan of the individual; such a pattern of repeated regeneration of a single skeletal element has not previously been documented. Given the position of these spines on the elevated crown of a moderately long-stemmed crinoid, frequent snipping by predatory fishes seems the most likely interpretation for the observed pattern. The repeatedly regenerated specimens occur in an assemblage displaying an unusually high regeneration frequency among pirasocrinid brachial spines, with over 30% of spines displaying at least one plane of regeneration. Paradoxically, pirasocrinids are unfavorable as prey items relative to other organisms and co-occurring crinoids; hence, it is most likely that associated biota (e.g., commensals, parasites) were the true targets of predators. This assemblage supports the interpretation that the development of extreme spinosity in pirasocrinids was largely driven by predation. However, this morphological pattern may largely represent an evolutionary strategy rooted in minimizing collateral damage incurred by predation on other organisms rather than direct predation on the crinoids themselves.

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